The basic operation and structure of a land mobile radio system is well known. Land mobile radio systems typically comprise one or more radio communication units (e.g., vehicle-mounted mobiles, portable radios, or dispatch consoles in a land mobile system and radio/telephones in a cellular system) and one or more repeaters that transceive information via RF communication resources. These communication resources, which may be used as voice and/or data resources, may be narrow band frequency modulated channels, time division multiplex slots, carrier frequencies, frequency pairs, spreading patterns, wireline telephone lines, and so forth. Land mobile radio, systems may be organized as trunked communication systems, where a plurality of communication resources is allocated amongst a group of users by assigning the repeaters on a communication-by-communication basis within an RF coverage area. Land mobile radio systems may be organized as conventional (or non-trunked) radio systems, where communication resources are dedicated to one or more groups of users.
Typically, a multiple-site trunked RF communication system includes communication units, communication sites, communication resources, and a communication resource allocator, also known as a central controller, site controller, or zone controller, which may reside within base stations or repeaters located at the communication sites. Each site has a substantially distinct coverage area and is geographically located within the system. A number of communication resources are assigned to each site, where at least one of the communication resources is used as a control channel, while the remaining communication resources are used as voice and/or data channels. The base stations or repeaters are used to generate the communication resources and are located at the site. Some communication systems are wide-area communication systems and comprise many communication sites, which may be grouped into communication zones to more effectively allocate the communication resources among the communication units.
The communication units, also called subscribers or subscriber units, are often arranged in talkgroups and may be located anywhere within the system (in the coverage area of any site) when communicating within the system. When a communication unit from a talkgroup requests a group call, the communication unit transmits, via the control channel of the site in which it is located, an inbound signalling word (ISW) to the central controller or other controlling entity. A group call allows all members of the same talkgroup that are located within the system to communicate with each other. The ISW generally comprises the requesting communication unit's individual identification number, the requesting communication unit's talkgroup, and a request for a group call. The ISW may contain more or less information, depending on the system.
Dispatch consoles may also be part of the system and are connected to the other fixed portions of the system (i.e., the infrastructure) via wire connections. Dispatch consoles may each control a multiplicity of communications resources or talkgroups and may communicate on these communications resources singly or simultaneously. In trunked systems, dispatch consoles request calls, similarly to the way communication units such as mobiles or portables issue ISWs, by making a PTT (Push-To-Talk) request and awaiting a PTT grant that gives the console permission to communicate and gives it an indication of which communication resource should be used for the communication. Similar to ISWs, the PTT request generally is comprised of the requesting console's individual identification number for a talkgroup, the desired talkgroup, and the request for a group call. The PTT request may also contain other information, depending upon the system type and the type of group call being requested.
Upon receiving the ISW, the central controller determines if there is an available communication resource to allocate for each site in which a member of the talkgroup presently is registered. If a communication resource is available in each of these sites, the central controller will place the group call and transmits an outbound signalling word (OSW) to the requesting communication unit and each member of the talkgroup. If a communication resource is not available in each of these sites, the central controller will not place the group call and instead transmits an outbound signalling word (OSW) to the requesting communication unit. The central controller may try to convert this request into an active call at a later time when resources are available. The central controller treats PTT requests similarly. If all of the necessary communication resources are available, the central controller issues a PTT grant. If a communication resource is not available at each of the required sites, the central controller issues a PTT busy to the dispatch console for the busy communication resource. Because dispatch consoles may transmit on multiple communication resources simultaneously, a console may receive PTT grants for some communication resources and PTT busies for other communication resources, even though the dispatcher desires the same audio message be sent on all communication resources. A busy may also result when a console attempts to use a resource already being used by another console. As a result, a portion (or all) of the call may be missed, the dispatcher may abort the call, or the dispatch may manually retransmit the message on the previously-busy communication resources.
When a mobile or portable communication unit is active in a group or individual call, and the communication unit travels out of the coverage area of its current site, the call needs to be handed off to another site for completion. A communication resource may not be available at the site, for example due to a high volume of calls, and a busy OSW may be issued to the subscriber unit. If a very high volume of calls is present at the time, the call may be completed before a new channel is assigned, hence the user will miss the completion of the call. In other situations, a call may be initiated, and a communication resource involved in the call may be removed from the call, for example, due to an emergency. Thus, any communication units engaged in the call on that communication resource will no longer be able to participate in the call while the resource is removed from the call. In addition, when a call requires a very large number of resources, it may be desirable to begin a communication with some communication resources, and add the additional communication resources as they become available, rather than waiting for resources for all of the subscriber units to participate in the call, so that communication throughput may be increased. Such a procedure, however, results in some users missing at least part of a call. For many users, such loss of communication is unacceptable.
Accordingly, there is a need for a method and apparatus that provides communication completion to all communication units, particularly for dispatch calls, even when busied or unavailable communication resources prevent such communication.